1. Field of the Invention
The present invention relates to method and apparatus for fabricating a single crystal of a compound semiconductor according to the vertical Bridgman method of growing a single crystal by gradually solidifying a semiconductor melt from a lower part to an upper part in a crucible.
2. Description of Related Art
In recent years, attention is being paid to the vertical Bridgman method as a method of obtaining a large GaAs crystal having the diameter exceeding 3 inches and a low dislocation density. According to the vertical Bridgman method, a crucible for containing a material and a seed crystal (seed) is placed on the inside of a heater of a vertical electric furnace and the material in the upper part is melt. After that, the seed is dipped into the melt and a single crystal is grown in the crucible from a lower part to an upper part, that is, from a lower part on the seed crystal side to an upper part.
The method of growing a crystal by moving the crucible and the heater relatively to each other is called the VB (vertical Bridgman) method. A method of growing a crystal by providing a temperature gradient in which the temperature is high in the upper part and is low in a lower part and decreasing the temperature as a whole while maintaining the temperature gradient constant is called a VGF (vertical gradient freezing) method.
In case of growing a single crystal of GaAs, a method of floating B2O3 on the surface of the material in order to prevent dissociation of As or a method of encapsulating the entire crucible in a quartz ampoule and growing a single crystal while maintaining the pressure at 1 atm which is a dissociation pressure of As in GaAs in the ampoule is employed.
In any case, as a heater of a part for controlling the interface between solid and melt, a heater which continues uniformly without a gap in the circumferential direction, that is, a heater having a circular shape in cross section is used for the following reason. For example, when there is variation in temperature in the circumferential direction, the interface between solid and melt becomes flat. It is therefore considered that variation in the electrical characteristics of the plane of a wafer which is cut from the crystal becomes large.
As a result of wholehearted studies of the inventors of the present invention, it was found that when the rounded heater is used, the interface between solid and melt tends to become recessed as a whole and there is a drawback that the yield of single crystal growth deteriorates.
It is, therefore, an object of the present invention to provide method and apparatus for fabricating a single crystal according to the vertical Bridgman method which solves the above problem of the recessed interface between solid and melt and can obtain a stable yield of single crystal growth.
In order to achieve the object, the invention is constructed as follows.
(1) There is provided a method of fabricating a single crystal of a compound semiconductor according to the vertical Bridgman method of growing a single crystal by arranging a crucible on the inner side of a heater of a vertical electrical furnace and gradually solidifying a semiconductor melt from a lower part to an upper part in the crucible, wherein a part for discharging the heat of the crucible toward the outside in the radial direction is formed at least in a part in the circumferential direction of a heater part for controlling the interface between solid and melt in the above heater which surrounds the crucible and the semiconductor melt is gradually solidified from a lower part to an upper part in the crucible while discharging the heat of the crucible not only in the vertical direction but also to the outside in the radial direction, thereby growing a single crystal.
The vertical Bridgman method includes the vertical gradient freezing method (VGF method) of growing a crystal only with temperature decrease, the vertical Bridgman method (VB method) of growing a crystal by relatively descending a growth vessel, a method of controlling the As pressure, and a method of preventing vaporization of As by covering the surface of the melt with B2O3.
When the heat of the crucible is allowed to flow not only in the vertical direction but also in the lateral direction, that is, to the outside in the radial direction (circumferential direction), the interface of the part which is cooled in the circumferential direction precedes and the interface of the part facing the heater is delayed. When the shape of the interface between solid and melt is seen from the growth direction, therefore, the part of the recessed face and the part of the projected face mixtedly exist in the interface between solid and melt. In the shape, a crystal defect does not easily occur and the yield of single crystal growth largely increases as compared with the case where the face is recessed as a whole.
(2) There is also provided a method of fabricating a single crystal of a compound semiconductor according to the vertical Bridgman method of growing a single crystal by arranging a crucible on the inner side of a heater of a vertical electrical furnace and gradually solidifying a semiconductor melt from a lower part to an upper part in the crucible, wherein a part for discharging the heat of the crucible toward the outside in the radial direction is formed at least in a part in the circumferential direction of a heater part for controlling the interface between solid and melt in the heater which surrounds the crucible and the semiconductor melt is gradually solidified from a lower part to an upper part in the crucible while maintaining the interface between solid and melt in a saddle shape.
When the interface is formed in a saddle shape and is seen from the growth direction, the part of the recessed face and the part of the projected face mixedly exist in the interface between solid and melt. In the shape, the crystal defect does not easily occur and the yield of single crystal growth largely increases as compared with the case where the interface has a recessed face as a whole.
Such a saddle-shaped interface can be obtained by forming a part for discharging the heat of the crucible toward the outside in the radial direction at least in a part in the circumferential direction of the heater part for controlling the interface between solid and melt in the heater surrounding the crucible. That is, by allowing the heat of the crucible to flow not only in the vertical direction but also in the circumferential direction (lateral direction), the interface of the part which is cooled in the circumferential direction precedes and the interface of the part which faces the heater is delayed, so that the saddle-shaped interface between solid and melt as shown in FIG. 1 can be realized.
(3) In each of the above methods, it is preferable to provide the parts for discharging the heat of the crucible toward the outside in the radial direction at two positions in the circumferential direction in the heater part for controlling the interface between solid and melt symmetrically with respect to the diameter direction. Thus, the interface can be formed in a symmetrical saddle shape.
(4) In each of the above methods, preferably, the part for discharging the heat of the crucible toward the outside in the radial direction is made by gaps of divided halves of the heater part which controls the interface between solid and melt. Although it is easy means, the interface can be formed in a saddle shape. The gaps in the circumferential direction of the halves of the heater are, in other words, the parts in which no heater exists in the circumferential direction and the heat of the crucible is discharged from the parts where there is no heater. By dividing the heater part into halves, the parts for discharging the heat of the crucible toward the outside in the radial direction exist in the symmetrical positions with respect to the diameter direction, so that the interface can be formed in a symmetrical saddle shape.
(5) In each of the above methods of fabricating a single crystal, the part for discharging the heat of the crucible toward the outside in the radial direction can be also realized by a cooling means provided in a part in the circumferential direction of the heater part for controlling the interface between solid and melt.
(6) There is also provided an apparatus for fabricating a single crystal of a compound semiconductor according to the vertical Bridgman method of growing a single crystal by arranging a crucible on the inner side of a heater of a vertical electrical furnace and gradually solidifying a semiconductor melt from a lower part to an upper part in the crucible, wherein at least a heater part for controlling the interface between solid and melt in the heater which surrounds the crucible is vertically divided into a plurality of parts and gaps for discharging the heat of the crucible to the outside in the radial direction are formed in a part in the circumferential direction of the heater part to allow the heat of the crucible to flow not only in the vertical direction but also from the gaps to the outside in the radial direction.
The gap in the circumferential direction of the heater is, in other words, a part where there is no heater in the circumferential direction and the heat of the crucible is discharged from the part where no heater exists. Consequently, the heat of the crucible is flowed not only in the vertical direction but also in the lateral direction, that is, to the outside in the radial direction (circumferential direction). The interface of the part which is cooled in the circumferential direction therefore precedes and the interface of the part which faces the heater is delayed. As a result, in the shape of the interface between solid and melt when it is seen from the growth direction, the part of the recessed face and the part of the projected face mixedly exist in the interface. In the shape, the crystal defect does not easily occur and the yield of single crystal growth largely increases as compared with at least the case where the interface has a recessed face as a whole.
(7) In the above apparatus, it is preferable that the heater part for controlling the interface between solid and melt is vertically divided into two parts. Although it is simple means, the interface can be formed in a saddle shape. The gap in the circumferential direction of the halves of the heater is, in other words, a part where there is no heater in the circumferential direction. The heat of the crucible is discharged from the part where there is no heater. By dividing the heater part into halves, the parts for discharging the heat of the crucible to the outside in the radial direction exist in positions which are symmetrical with respect to the diameter direction, so that the interface can be formed in a symmetrical saddle shape.
(8) There is also provided an apparatus for fabricating a single crystal of a compound semiconductor according to the vertical Bridgman method of growing a single crystal by arranging a crucible on the inner side of a heater of a vertical electrical furnace and gradually solidifying a semiconductor melt from a lower part to an upper part in the crucible, wherein cooling means for discharging the heat of the crucible to the outside in the radial direction are provided at positions symmetrical with respect to the diameter direction at least in a part in the circumferential direction of the heater part for controlling the interface between solid and melt in the heater which surrounds the crucible, so that the heat of the crucible flows not only in the vertical direction but also from the gaps to the outside in the radial direction.
By providing the cooling means as mentioned above, the heat of the crucible can be allowed to flow not only in the vertical direction but also in the lateral direction, that is, to the outside in the radial direction (circumferential direction). The interface of the part which is cooled in the circumferential direction precedes and the interface of the part which faces the heater is delayed. Consequently, the interface between solid and melt is formed in a shape where the part of the recessed face and the part of the projected face mixedly exist when it is seen from the growth direction. In the shape, the crystal defect does not easily occur and the yield of single crystal growth largely increases at least as compared with the case where the interface has a recessed face as a whole. The cooling means can be made by a part where a water-cooled tube is arranged or a part where an insulating material is removed.